This invention relates generally to electrical brushes for use in dynamoelectric machines, and more particularly to brushes for automatically maintaining a uniform commutator contact surface.
In dynamoelectric machines such as motors, generators, or alternators having brushes making electrical current conductive contact with a radial or cylindrical surface of a conductive structure such as a commutator or slip ring, the moving contact between the elements causes mutual wearing; firstly, by simple mechanical frictional wearing or abrasion; and secondly, by electrical erosion.
Mechanical wear is affected by such things as contact area, contact pressure, shock, vibration, and non-uniformity. To reduce mechanical wear of the conductive structure, the brushes are made of a softer material so that they are the principal wearing element rather than the commutator.
The major cause of wear on the conductive structure is that of electrical erosion. Especially susceptable to such action is a heavy-duty dynamoelectric machine operating with high current such as in the case of a locomotive traction motor. The electrical wearing which normally occurs, tends to erode material from the surface of the conductive structure, such as the commutator or slip ring along the contact path of the brush on the moving surface. This path tends to become rough and pitted due to even normal arcing over the area of instantaneous contact. Once begun, this roughening increases the arcing which, in turn, increases the rate of roughness increase in a mutually causative and progressively rapidly deteriorating fashion.
In addition to the pitting action, certain commutators develop a wavy pattern of wear, often characterized by four or six flat spots or dips. When this occurs, the higher portions on the circumference will often develop a thin, uniform and conductive film. Little or no arcing occurs on these high portions, but on the adjacent low portions the arcing is increased and a thick, non-conductive film is developed thereon which further inhibits conduction. Thus the commutator is progressively worn in a manner which causes it to be non-cylindrical in form. Other causes of non-uniformity of the commutator surface includes that of mechanical shifting caused by stresses on the commutator. The result is a relative lowering or raising of individual commutator bars which causes a variation in both electrical and mechanical erosion on the surface of the commutator.
A non-uniform commutator or slip ring surface will increase the mechanical wear of both the brushes and the conductive structure. If the non-uniformity is severe enough, chattering, or vibration of the brushes occurs and will eventually result in failure of the brushes.
Accordingly, during the life of such machines, periodic servicing is required to replace brushes and to dress the contact surface of the moving structure. The commutator is commonly trued to form by removing it from the machine and turning it in a lathe. Where the cost of removal is too great for such methods, some form of portable grinding wheel or lathe-turning tool is used without removal of the structure from the machine. In either case, the machine must be idle for a considerable length of time.
A method of automatically resurfacing the commutator during normal operation of the machine was devised wherein an abrasive brush, or a brush containing an abrasive material throughout its structure, was installed in the standard brushholder. This could be either a temporary arrangement, or a continuous arrangement wherein the abrasive element performed the normal function of a standard brush. Both arrangements tended to cause excessive wear of the commutator and neither arrangement provided corrective action to out-of-round conditions, since high portions were not selectively worn at a greater rate than low portions. In fact, mechanical vibration and brush response usually deepened any existing low portions of the commutator and thereby increased the roughness.
An alternate arrangement provided for automatic periodic abrasive wear on the commutator by the interpositioning in longitudinal spaced relationship of transverse discontinuities between successive like portions in the brush structure. Such an arrangement thus conserves armature material by not continually abrading the surface, but as in the afore-mentioned methods, no corrective action to out-of-round conditions is afforded.
It is, therefore, an object of this invention to provide a new and improved electrical brush that will substantially reduce periodic servicing and machine down time.
Another object of this invention is the provision for electrical brushes which significantly reduce electrical erosion of the commutator used in combination therewith.
Yet another object of this invention is the provision for the automatic self-rounding of a machine commutator.
Still another object of this invention is the provision for selective abrasion of a commutator surface to produce and maintain a round commutator profile.
A further object of this invention is the provision for an electrical brush which is economical to manufacture and functional and durable in use.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.